Add Power Query (M) language support

[jacob-kraniak]

Description

This pull request adds support for the Power Query (M) language (also known as Power Query Formula Language).

• Added language definition in lib/linguist/languages.yml
• Registered the official syntax highlighting grammar
• Included representative real-world sample files

This implements the addition discussed and approved in issue #7856.

Checklist:

• I am adding a new language.
  • The extension of the new language is used in hundreds of repositories on GitHub.com.
    • Search results for each extension:
      • .pq: https://github.com/search?q=NOT+is%3Afork+path%3A*.pq+NOT+path%3A*.pqm&type=code
      • .pqm: https://github.com/search?q=NOT+is%3Afork+path%3A*.pqm&type=code
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • Samples exported from my own repository: https://github.com/jacob-kraniak/MXtoolbox-Powerquery
      • Specifically: samples/PowerQuery/fnMxToolboxLookup.pq (custom function) and samples/PowerQuery/q_MxToolboxScore_SOAR3.pq (multi-step query)
    • Sample license(s):
      MIT License. The source repository is licensed under MIT, and the extracted sample files are released under the same terms that cover the Linguist project.
  • I have included a syntax highlighting grammar: https://github.com/microsoft/powerquery-language
  • I have added a color
    • Hex value: #d38e0d
    • Rationale: Gold tone inspired by Power BI / Power Query branding. Provides strong contrast in both light and dark GitHub themes and is consistent with other Microsoft data languages.
  • I have updated the heuristics to distinguish my language from others using the same extension.
    (Not required — .pq and .pqm show low conflict risk with other languages.)

[DecimalTurn]

Looking good, just 2 comments.

[DecimalTurn]

LGTM